Effect of Different Zinc Species on Mn-Ce/CuX Catalyst for Low-Temperature NH₃-SCR Reaction: Comparison of ZnCl₂, Zn(NO₃)₂, ZnSO₄ and ZnCO₃

Abstract

The effects of four distinct zinc species (ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃) on a Mn-Ce co-doped CuX (MCCX)catalyst were investigated and contrasted in the low-temperature NH₃-SCR process. Aqueous solutions of ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃ were used to poison the catalysts. The catalytic activity of all catalysts was assessed, and their physicochemical properties were studied. There was a notable drop trend in catalytic activity in the low temperature range (200 °C) after zinc species poisoning on MCCX catalyst. Interestingly, ZnSO₄ and ZnCO₃ on MCCX catalyst had more serious effect on catalytic activity than Zn(NO₃)₂ and ZnCl₂ from 150 °C to 225 °C, in which NO conversion of the MCCX-Zn-S and MCCX-Zn-C catalysts dropped about 20–30% below 200 °C compared with the fresh MCCX catalyst. The zeolite X structure was impacted by Zn species doping on the MCCX catalyst, and the Zn-poisoned catalysts had less acidic and lower redox ability than fresh Mn-Ce/CuX catalysts. Through the results of in situ DRIFTS spectroscopy experiments, all catalysts were governed by both Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms, and the possible mechanism for poisoning the Mn-Ce/CuX catalyst using various zinc species was revealed.

1. Introduction

The worldwide deterioration in air quality has led to the establishment of stronger rules on the emission of nitrogen oxides (NO_x) [1,2]. The selective catalytic reduction (SCR) of NO_x with NH₃ is a well-established and widely utilized technology [3]. According to a number of studies [4,5,6], despite its popularity, the commercial V₂O₅-WO₃(MoO₃)/TiO₂ catalyst has a few limitations that prevent it from being truly helpful. It is hazardous to living things because of vanadium species, does not work well at lower temperature, and can only be used effectively between 350 and 450 °C. This shows how important it is to look into low-temperature SCR catalysts that do not use vanadium to control NO_x emissions.

Compared to vanadium-based catalysts, copper-based catalysts have better redox properties and thermodynamic stabilities [7,8], making them a viable alternative. It was discovered by Wang et al. [9] that the maximum NO conversion of the Cu-ZSM-5 catalyst occurred between 200 and 400 °C, with practically full conversion, because of the highly preferential production of active copper species and the quick flip between Cu²⁺ and Cu⁺ species during the NH₃-SCR process. Cu-exchanged zeolite catalysts have recently been the subject of extensive study in NH₃-SCR reaction [10,11]. Tarach et al. [12] compared the activity and stability of Cu-exchanged zeolites with different framework topologies in NH₃-SCR reaction and found that the Cu-ZSM-5 catalyst displayed an extraordinary activity in the NH₃-SCR process, achieving nearly 100% NO conversion at 175 °C with almost 100% N₂ selectivity. Our prior research [13,14] found that zeolite X could be generated cheaply from the blast furnace slag, making ion-exchange techniques a viable option for producing CuX zeolite catalyst. Doping Mn and Ce oxides on CuX zeolite could improve its SO₂ + H₂O resistance and enhance its low-temperature NH₃-SCR activity. Generally, the catalysts nevertheless often experienced severe poisoning by alkali, alkaline earth, and heavy metals in the actual fuel gas state. The deactivation rate of various poisonous zinc species was shown to be in the order of ZnCl₂ > ZnSO₄ > ZnO by Wang et al. [15], who studied the effects of three distinct zinc species (ZnO, ZnSO₄, and ZnCl₂) on Sb-CeZr₂O_x catalysts. According to our earlier research [16,17], ZnCl₂ had a more severe poisoning effect on the Mn-Ce/AC catalyst than ZnSO₄, and Zn²⁺ would occupy the acidic site of the catalyst after loading zinc salts. Nonetheless, the poisoning impact of various zinc species on Cu-exchanged zeolite catalysts has received insufficient attention till now.

The current research looked into how different zinc ions affected the catalytic performance of the Mn-Ce/CuX catalyst in low-temperature denitration reactions. To poison the Mn-Ce/CuX catalyst, we used an impregnation technique with ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃ solutions. The causes of zinc salt poisoning and deactivation of the Mn-Ce/CuX catalyst have been identified, and the possible mechanism for poisoning the Mn-Ce/CuX catalyst using various zinc salts has been proposed, by comparing the catalytic activity, dispersion degree of active component, surface morphology and pore structure, surface acidity, redox performance, element concentration of active component, and surface reaction path of the catalyst before and after poisoning.

2. Results and Discussions

2.1. Denitration Performance

The catalytic activity for fresh MCCX and catalysts poisoned with different Zn species (MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts) can be found in Figure 1. NO conversion of Zn-poisoned catalysts is much lower than that of the fresh MCCX catalyst at temperatures below 200 °C, as seen in Figure 1a. In detail, NO conversion of the MCCX-Zn-C catalyst surpassed the other three poisoned catalysts at temperatures below 125 °C, whereas NO conversion of the MCCX-Zn-Cl catalyst outperformed the other three Zn-poisoned catalysts at temperatures over 150 °C, reaching nearly 100% at 275 °C. The NO conversion rate for the MCCX-Zn-S and MCCX-Zn-C catalysts dropped by about 20–30% below 200 °C, making it the worst performing catalyst over the whole reaction temperature range. It was discovered that the conversion of the catalysts is most affected by the different Zn species in the lower temperature range, whereas they have almost no effect at higher temperatures. According to Figure 1b, comparing the N₂ selectivity of the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts to that of the fresh MCCX catalyst, there was an approximate 20% improvement. Figure 1c displays the results of a measurement of the concentration of NH₃ and N₂O in both Zn-poisoned and new MCCX catalysts. Zn-poisoned catalysts had lower concentrations of N₂O and higher concentrations of NH₃ compared to the fresh MCCX catalyst, among which the MCCX-Zn-S sample also had more NH₃ than N₂O.

2.2. Structural Properties

X-ray diffraction (XRD) patterns of MCCX, MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts are shown in Figure 2. The zeolite X structure was described by peaks at 2theta = 6.8°, 15.5°, 18.5°, 20.2°, 23.4°, 26.8°, and 30.5°, while the CuO phases were characterized by peaks at 2theta = 35.5°, 38.9°, 48.8°, and 61.5°. It was discovered that the peak intensity of zeolite X on Zn-poisoned catalysts was lesser than on the fresh catalyst, whereas the peak intensity of CuO phase on Zn-poisoned catalysts was stronger than on the fresh one. It was noted that the peak intensity of zeolite X on the MCCX-Zn-Cl catalyst was much stronger than the other three catalysts, while the peak intensity of zeolite X on MCCX-Zn-S and MCCX-Zn-C catalysts were weaker. This result suggested that the zeolite X structure was impacted by Zn species doping on MCCX catalyst, and the zinc species contributed to the migration of CuO species. The absence of the Zn species peak indicates that they are uniformly dispersed on the catalyst surface. Micrographs of the catalysts (Figure 3) show that Zn-poisoned catalysts had a zeolite X structure that deviated from the usual octahedron, and some of the zeolite X particles agglomerated. In particular, the zeolite X structure of the MCCX-Zn-C catalyst had more serious damage than the other three catalysts. Furthermore, EDS mapping revealed that Cu aggregated on catalysts due to Zn poisoning, leading to an insufficiency of Cu active sites.

2.3. XPS Analysis

To find out more about the elemental compositions of the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts, XPS spectra of Cu 2p, O 1s, Mn 2p, and Zn 2p were measured, as displayed in Figure 4. Peaks at the 948–966 eV, 940–945 eV, and 930–948 eV regions in the Cu 2p spectra (Figure 4a) were attributed to Cu 2p_1/2, satellite peaks, and Cu 2p_3/2, respectively [11,12]. Additionally, the spectra of Cu 2p_3/2 could be divided into two peaks; one at 935 eV was attributed to Cu²⁺, and another at 933 eV was attributed to Cu⁺ ions [9,10,18,19]. By calculating the area of the corresponding peaks, we were able to determine the relative abundance of the different Cu species and establish the following ranking for the concentration of Cu²⁺ species: the MCCX-Zn-S catalyst (45.1%) was more effective than the MCCX-Zn-N catalyst (43.5%), MCCX-Zn-Cl catalyst (39.7%), and MCCX-Zn-C catalyst (37.1%). In general, the isolated state of Cu²⁺ species in the zeolite lattice was an active site during the catalytic process [8,20,21].

Figure 4b shows the XPS spectra of Mn 2p for MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts. Mn 2p_3/2 and Mn 2p_1/2 were responsible for the two large peaks seen at about 642 and 650 eV, respectively [22]. In addition, three fitting peaks at about 641.5 eV, 640.5 eV, and 639.2 eV were identified in the spectra of Mn 2p_3/2, which corresponded to Mn⁴⁺, Mn³⁺, and Mn²⁺ species, respectively [23,24,25]. The concentration order of species Mn4+ was as follows: MCCX-Zn-N > (32.5%) > MCCX-Zn-Cl (30.7%) > MCCX-Zn-C (26.8%) > MCCX-Zn-S (22.5%) catalysts. Previous studies indicated that high valve state Mn species (Mn⁴⁺ species) facilitated NO oxidation to NO₂ and accelerated the “fast SCR” process at low temperatures [26,27].

O 1s spectra of MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts were depicted in Figure 4c. Three fitting peaks at around 531.6 eV, 529.5 eV, and 533.5 eV belonged to chemosorbed oxygen species (O_α), lattice oxygen species (O_β), and chemisorbed oxygen or weakly bound oxygen species (O_γ), respectively [28,29,30]. Generally, the catalysts exhibited the increased NH₃-SCR activity at higher concentrations of chemosorbed oxygen species due to their greater mobility in comparison to the other two oxygen species [13,23,31]. The concentration order of species O_α was as follows: MCCX-Zn-S (66.7%) > MCCX-Zn-C (57.1%) > MCCX-Zn-Cl (55.8%) > MCCX-Zn-N (50.5%) catalysts. The SO₄²⁻ and CO₃²⁻ might provide more chemosorbed oxygen species on catalysts, which was in line with the previous study [32]. In Figure 4d, there is one peak belonging to Zn 2p_3/2 that could be found at Zn-poisoned catalysts [33]. The weakest peak intensity of the four catalysts was observed in the MCCX-Zn-S catalyst from the summary XPS spectra displayed in Figure 4e, attesting to the low amount of the surface’s active components and, consequently, lesser SCR activity.

2.4. Acidity and Redox Property

The main factor in SCR activity is the acidity of the catalyst’s surface. Figure 5a–c shows the NH₃-TPD results and in situ DRIFTS spectra of NH₃ desorption at temperatures between 35 and 300 °C for MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts. Figure 5a shows that all Zn-poisoned catalysts exhibit several fitting peaks between 100 and 800 °C. The peak at 122 °C is associated with low acidity, those between ca. 200 and 400 °C are linked to medium acidity, and those beyond 400 °C are associated with high acidity [14,32,34]. In addition, as shown in Figure 5b, the area of fitting peaks was used to determine the amount of adsorbed NH₃. All the catalysts were found to have a greater middle acidity, and the following list shows what they ranked in terms of how much NH₃ they were able to absorb: concentrations of MCCX-Zn-Cl (478.1 μmoL/g), MCCX-Zn-C (435.9 μmoL/g), MCCX-Zn-N (299.1 μmoL/g), and MCCX-Zn-S (219.8 μmoL/g). Figure 5c shows the in situ DRIFTS spectra of NH₃ desorption, which shows multiple bands on each of the four catalysts between 3000 and 3800 cm⁻¹. These bands were assigned to Cu²⁺ absorbed NH₃ species (3182 and 3332 cm⁻¹) and NH₄⁺ species on Brønsted acid sites (3272 cm⁻¹) [9,35]. Bands corresponding to coordinated adsorbed NH₃ species on Lewis acid sites (1610 and 1260 cm⁻¹) and NH₄⁺ species on Brønsted acid sites (1390 cm⁻¹) were also observed [36,37]. The acidity reduced as the reaction temperature increased, which can be seen from the gradual dropping in peak intensity of the acid sites. Lewis acid sites on catalysts were also shown to be more stable than Brønsted acid sites, indicating that Lewis acid sites were the most important acid sites for the reactions.

Profiles of H₂-TPR for MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts are shown in Figure 6. The MCCX-Zn-Cl catalyst was found to have three fitting peaks at 200–400 °C, but the other three Zn-poisoned catalysts only had two fitting peaks at 200–400 °C. For the peak at ca. 200 °C, CuO was reduced to Cu⁰; for the peak at 250 °C, Cu²⁺ species on the hydroxyl group of zeolite X were reduced; and for the peak at 300 °C, isolated Cu²⁺ ions located in lattice zeolite X were reduced. The peak over 400 °C was related to the reduction of Cu⁺ ions, and no peak existed in the Zn-poisoned catalysts. Peaks at ca. 200–500 °C belonged to the reductions of MnO₂ to Mn₂O₃, Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO. The CeO₂ redox process of surface-capping oxygen occurs at temperatures of ca. 400 °C [38,39,40]. In addition, the consumption of H₂ for the Zn-poisoned catalysts was as follows: MCCX-Zn-C (6.6 mmoL/g) > MCCX-Zn-S (6.2 mmoL/g) > MCCX-Zn-Cl (5.5 mmoL/g) > MCCX-Zn-N (2.7 mmoL/g). It was discovered that the reduction of MCCX catalysts was affected by the presence of various Zn species, particularly the isolated Cu²⁺ ions in lattice zeolite X and Cu⁺ ions. It could be found that the Zn poisoning produced more copper oxides on MCCX-Zn-S and MCCX-Zn-C catalysts, and reduced the Cu²⁺ active sites.

2.5. In Situ DRIFTS Studies

2.5.1. NO + O₂ Reacting with Pre-Adsorbed NH₃ Species

In order to study the impact of various Zn species on surface acidity types of the catalysts, the in situ DRIFTS spectra of MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts for NH₃ adsorption at 200 °C were investigated. As shown in Figure 7, the bands of Cu²⁺-absorbing NH₃ species at 3182 and 3332 cm⁻¹, NH₄⁺ species on Brønsted acid sites at 3272 cm⁻¹ and 1390 cm⁻¹, and coordinated adsorbed NH₃ species on Lewis acid sites at 1610 cm⁻¹ and 1260 cm⁻¹ [9,35] gradually increased in intensity after NH₃ was introduced for 5 min. It could be found that the bands belonging to Lewis acid sites were much stronger than that of Brønsted acid sites on Zn-poisoned catalysts.

The intermediates of the reaction between NO + O₂ and pre-adsorbed NH₃ species at 200 °C were studied by performing in situ DRIFTS measurements over MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts, as shown in Figure 8. When NH₃ was introduced, bands with intensities of Cu²⁺-absorbing NH₃ species (3182 and 3332 cm⁻¹), NH₄⁺ species on Brønsted acid sites (3272 cm⁻¹ and 1390 cm⁻¹), and coordinated adsorbed NH₃ species on Lewis acid sites (1610 cm⁻¹ and 1260 cm⁻¹) were observed on the four Zn-poisoned catalysts, and these bands disappeared after 20 min of NO + O₂ purging. After that, many striations, which could be attributed to various nitrate and nitrite species, began to show up. Nitrate species at the Cu²⁺ active site were attributed to the 1915 cm⁻¹ band [41], while those at 1600–1650 cm⁻¹ were characterized as bidentate bridging nitrate (M-O₂-NO), those at 1500–1570 cm⁻¹ were described as bidentate chelating nitrate (M-O₂NO), those at 1480–1530 cm⁻¹ as monodentate nitrate (M-O-NO₂), and those at 1317–1400 cm⁻¹ as free-NO₃⁻ species of antisymmetric N-O stretches [35,42,43,44]. Bidentate nitrite (M-O₂N) and bridging nitrite (M-O₂-N) contributed the bands at 1320 cm⁻¹ and 1575 cm⁻¹, respectively [42,45]. It indicated that both the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms were at play in all of the catalysts. ZnCl₂ species on MCCX catalysts had a slight influence on the Cu²⁺ active site, while the band intensity of nitrate species on the Cu²⁺ active site for MCCX-Zn-S was weaker than that of the other three catalysts.

2.5.2. NH₃ Reacting with Pre-Adsorbed NO + O₂ Species

Figure 9 displays the in situ DRIFTS spectra of NO + O₂ co-adsorption at 200 °C over MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts. The bands of bidentate bridging nitrate (M-O₂-NO) (at 1600–1650 cm⁻¹), bidentate chelating nitrate (M-O₂NO) (at 1500–1570 cm⁻¹), monodentate nitrate (M-O-NO₂) (at 1480–1530 cm⁻¹), and the free-NO₃⁻ species (at 1300–1400 cm⁻¹) of antisymmetric N-O stretches appeared, and the bands of nitrate species on Cu²⁺ active site (at 1915 cm⁻¹) appeared over the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts [35,42,43,44,45]. The longer they were exposed to NO + O₂, the stronger their bands became. It was found that the nitrate species adsorbing on the Cu²⁺ active site over the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts had much stronger band intensity than the other bands of nitrites and nitrates, suggesting that the Cu²⁺ active site was the primary site involved in the NO + O₂ adsorption. Further, the nitrate and nitrite intermediate products were significantly affected by the poisoning impact of ZnCl₂, Zn(NO₃)₂, and ZnSO₄ on MCCX catalysts, as compared to ZnCO₃.

Figure 10 displays the in situ DRIFTS measurements performed over the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts at 200 °C to investigate the reaction intermediates between NH₃ and pre-adsorbed NO + O₂ species. Nitrate species on the Cu²⁺ active site (Cu(NO₃)_x), bidentate bridging nitrate (M-O₂-NO), bidentate chelating nitrate (M-O₂NO), monodentate nitrate (M-O-NO₂), NH₄NO₃ species, and the free-NO₃⁻ species of antisymmetric N-O stretches all showed gradual decreasing trends in band intensity when NH₃ was introduced, which eventually disappeared after 15 min. Then, bands appeared at increasing intensities with increasing time of NH₃ exposure, which were attributed to Cu²⁺-absorbing NH₃ species (3182 and 3332 cm⁻¹), NH₄⁺ species on Brønsted acid sites (3272 cm⁻¹), and coordinated adsorbed NH₃ species on Lewis acid sites (1610 cm⁻¹ and 1260 cm⁻¹).

3. Discussion

From the XRD results, it could be found that the zeolite X structure was impacted by Zn species doping on the MCCX catalyst, and the zinc species contributed to the migration of CuO species on the Zn-poisoned catalysts. Some of the zeolite X particles agglomerated, and Cu species aggregated on catalysts due to Zn poisoning, leading to insufficient Cu active sites from the SEM and EDS results. Furthermore, the zeolite X structure for the MCCX-Zn-S and MCCX-Zn-C catalysts had more serious damage. From the XPS analysis, it could be found that the MCCX-Zn-C catalyst had less Cu²⁺ species, while the MCCX-Zn-S catalyst had less Mn⁴⁺ species. The Cu²⁺ species and Mn⁴⁺ species were the main active sites for low-temperature SCR reaction, which might be the reason that the ZnSO₄ and ZnCO₃ on the MCCX catalyst had a more serious effect on catalytic activity. The MCCX-Zn-Cl catalyst had the strongest surface acidity, while the MCCX-Zn-S catalyst had the weakest surface acidity, which affected the adsorption of NH₃. From the H₂-TPR analysis, it could be found that Zn poisoning produced more copper oxides for MCCX-Zn-S and MCCX-Zn-C catalysts and reduced the Cu²⁺ active sites, thereby having an influence on NO and NH₃ adsorption. The bands belonging to Lewis acid sites were much stronger than that of Brønsted acid sites on Zn-poisoned catalysts from the in situ DRIFTS spectra of NH₃ adsorption. Based on the in situ DRIFTS spectra of NO + O₂ and NH₃ species, both the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanisms were at play in all of the catalysts. ZnCl₂ species on MCCX catalysts had a slight influence on the Cu²⁺ active site, while the band intensity of nitrate species on the Cu²⁺ active site for MCCX-Zn-S was weaker than that of the other three catalysts. The scheme of mechanism effect for different zinc species on the Mn-Ce/CuX catalyst is summarized in Figure 11.

4. Materials and Methods

4.1. Catalysts Preparation

The Mn-Ce/CuX catalysts utilized in this study were synthesized using the impregnation method, with the Mn-Ce mixed oxides loading of 5 wt.% (molar ratio of Mn/Ce was 7:3), and the details of the preparation process were described in our earlier study [19]. The first synthesis of CuX catalyst included both hydrothermal and ion exchange methods [13]. After obtaining CuX catalysts, they were added into a precursor solution including a predetermined amount of manganous nitrate and cerous nitrate, and the resulting slurry was dried out in a water bath at 80 °C. The Mn-Ce/CuX catalyst was produced by drying in an oven for 10 h at 80 °C and then calcining them in a muffle furnace for 3 h at 500 °C in air condition.

A total of 1 wt.% Zn species was used to poison the Mn-Ce/CuX catalysts, which were then composited using an impregnation approach in a variety of zinc compounds which were ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃, respectively. After evaporating the solutions of ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃ in an 80 °C water bath, the samples were dried in an 80 °C oven and then calcinated for 3 h at 500 °C in air. MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C were the names assigned to the several Zn-poisoned catalysts for ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃.

4.2. Catalytic Activity Tests

The NH₃-SCR activity of the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts was studied in a 10 mm diameter quartz reactor using 0.5 g (40–60 mesh) samples. The temperature went from 75 to 300 °C in increments of 25 °C, and each temperature point was maintained for 30 min. The simulant mixed gas contained 500 ppm NH₃, 500 ppm NO, 11 vol% O₂, and N₂ as balanced gas with a total flow gas of 100 mL/min and an hourly space velocity of 36,000 h⁻¹. The amount of NO, NH₃, NO₂, and N₂O coming out was measured by a gas analyzer (Thermo Scientific, Antaris IGS, Waltham, MA, USA). The NO conversion and N₂ selectivity of the MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts were determined using the following equations (Equations (1) and (2)) [27,37].

$$\mathrm{NO} \mathrm{conversion} \left(\%\right)=(1-\frac{{\left[\mathrm{NO}\right]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}}) \times 100\%$$

(1)

$${\mathrm{N}}_2 \mathrm{selectivity} (\%)=(1-\frac{{2[\mathrm{N}}_2{\mathrm{O}]}_{\mathrm{out}}-{{[\mathrm{NO}}_2]}_{\mathrm{out}}}{{\left[\mathrm{NO}\right]}_{\mathrm{in}}{+[\mathrm{NH}}_3{]}_{\mathrm{in}}-{\left[\mathrm{NO}\right]}_{\mathrm{out}}-{ [\mathrm{NH}}_3{]}_{\mathrm{out}}}) \times 100\%$$

(2)

4.3. Catalyst Characterization

The X-ray diffraction (XRD) patterns were conducted to examine the composition and crystallinity of chemicals on the catalyst surface using the Rigaku D/max-2500 PC diffractometer which was recorded with a 2θ range of 5–90° at a scanning speed of 5°/min. Scanning electron microscope (SEM) analysis was used to investigate the surface microstructure of MCCX-Zn-Cl, MCCX-Zn-N, MCCX-Zn-S, and MCCX-Zn-C catalysts by the Thermo Scientific Quattro SEM. The content and distribution of surface elements were recorded using energy dispersive X-ray spectrometry (EDS) mapping.

X-ray photoelectron spectroscopy (XPS) experiments were used to measure the Cu 2p, O 1s, Mn 2p, and Zn 2p by the Thermo ESCALAB 250Xi with Al Kα radiation (1484.6 eV) at room temperature. The binding energy of the surface elements of the sample was calibrated using C 1s (284.6 eV), and the sample was degassed in a vacuum for 1 h before testing to minimize the influence of surface impurities.

A H₂-TPR test was used on an Auto Chem II TPR/TPD 2920 detector. The specific operation process was as follows: 150 mg of catalyst was placed in the U-shaped quartz reaction tube, and the sample was pretreated under Ar flow of 100 mL/min. Then, the sample was heated from room temperature to 300 °C at a heating rate of 5 °C/min and then cooled to 50 °C after purging for 1 h, after which the sample was heated to 600 °C (heating rate of 10 °C/min) under 5% H₂/Ar atmosphere (flow rate of 50 mL/min). At the same time, the detector recorded the TCD signal. NH₃-TPD tests were conducted by the Chembet Pulsar TPR/TPD detector (The American Konta Company), and the pretreated steps were the same as H₂-TPR. The sample was then exposed to a 5% NH₃/Ar atmosphere (flow rate of 100 mL/min) for 1 h and followed by purging in N₂ (100 mL/min). Then, the sample was heated to 800 °C at a constant heating rate of 5 °C/min under a flow of 100 mL/min Ar, and the TCD signal was detected.

In situ diffuse reflectance infrared Fourier transform (in situ DRIFTS) studies were investigated on the Thermo Fisher Nicolet iS50 spectrometer. Before the test, all the catalysts were purged at 300 °C for 30 min under N₂ and 11 vol% O₂ atmosphere (total flow rate of 100 mL/min); then, the background spectrum at a desired temperature was collected. Then, the catalysts were exposed under 500 ppm NH₃/N₂ or 500 ppm NO/N₂ and 11 vol% O₂ gas for 40 min (total flow rate of 100 mL/min), after which they were flushed under N₂ gas for 60 min.

5. Conclusions

This research investigated and contrasted the effects of four distinct zinc species (ZnCl₂, Zn(NO₃)₂, ZnSO₄, and ZnCO₃) on the Mn-Ce co-doped CuX catalyst in a low-temperature NH₃-SCR process. Poisoned catalysts were made by impregnating in aqueous solutions of zinc chloride, zinc nitrate, zinc sulfate, and zinc carbonate. All catalysts’ NH₃-SCR activity were quantified, and the physicochemical characteristics were studied. There was a notable drop in catalytic activity for the catalyst in the low temperature range after zinc species poisoning, with the order of poisoning going as follows: ZnSO₄ > ZnCO₃ > Zn(NO₃)₂ > ZnCl₂. Zn species affected the reduction of the MCCX catalyst, especially isolated Cu²⁺ ions in lattice zeolite X, leading to a decrease in the SCR activity. ZnSO₄’s poisoning effect on MCCX catalysts was much more pronounced on the nitrate and nitrite intermediate products than that of ZnCO₃, ZnCl₂, and Zn(NO₃)₂, and all the catalysts were governed by both the Langmuir–Hinshelwood (L–H) and Eley–Rideal (E–R) mechanism as evidenced by the in situ DRIFTS spectra.